Chemical vapor infiltration and deposition (CVI/CVD) is a well known process for depositing a binding matrix within a porous structure. The term “chemical vapor deposition” (CVD) generally implies deposition of a surface coating, but the term is also used to refer to infiltration and deposition of a matrix within a porous structure. As used herein, the term CVI/CVD is intended to refer to infiltration and deposition of a matrix within a porous structure. The technique is particularly suitable for fabricating high temperature structural composites by depositing a carbonaceous or ceramic matrix within a carbonaceous or ceramic porous structure resulting in very useful structures such as carbon/carbon aircraft brake disks, and ceramic combustor or turbine components.
Generally, manufacturing carbon parts using a CVI/CVD process involves placing preformed porous structures in a furnace and introducing a high temperature reactant gas to the porous structures. A variety of porous structures and reactant gases may be used, but typically, a fibrous carbon porous structure is used with a reactant gas mixture of natural gas and/or propane gas when carbon/carbon aircraft brake disks are manufactured. As well understood by those in the art, when the hydrocarbon gas mixture flows around and through the porous structures, some of the carbon atoms separate from the hydrocarbon molecules, thereby depositing the carbon atoms within the interior and onto the surface of the porous structures. As a result, the porous structures become more dense over time as more and more of the carbon atoms are deposited onto the structures. This process is sometimes referred to as densification because the open spaces in the porous structures are eventually filled with a carbon matrix until generally solid carbon parts are formed. U.S. Pat. Nos. 5,480,678 and 5,853,485 to Rudolph et al. and U.S. Pat. No. 6,669,988 to Daws et al., also describe in detail additional aspects of CVI/CVD processes.
Densification processes for annular brake disks may be characterized as either conventional isothermal densification processes or pressure gradient densification processes or variants thereof. In conventional isothermal densification, annular brake disks are arranged in stacks with adjacent brake disks stacked on top of each other. A center opening region is thus formed through the center of each stack. Typically, spacers are placed between adjacent brake disks to form open passages between the center opening region and the outer region. Thus, the reactant gas flows randomly around the stack and may flow through the open passages from the center opening region to the outer region or vice versa. As a result, the pressure differential between the inlet and outlet ducts of the furnace is usually relatively low in conventional isothermal processes.
In pressure gradient densification, the open passages between the center opening region and the outer region are sealed to constrict the flow of the reactant gas between the center opening region and the outer region. Therefore, the pressure differential between the inlet and outlet ducts of the furnace is higher than the pressure used in isothermal densification. As a result, the high pressure differential forces the reactant gas to flow through the interior of the porous brake disk structures, thereby increasing the rate of densification compared to isothermal processes. Conventional isothermal and pressure gradient densification processes may also be combined to achieve optimum densification. For example, a pressure gradient densification process may be used in a first densification to decrease densification time, and a conventional isothermal densification process may be used in a second densification to improve densification quality.
One area of concern during densification is the distribution of the reactant gas flow through and around the porous structures. Gas flow distribution can have a significant impact on the quality of the densified carbon parts and also can affect the cost of production. For example, in one method disclosed in U.S. Pat. No. 5,904,957 to Christin et al., stacks of annular preforms are placed in a furnace with spacer elements placed between each of the preforms and between the last preforms in the stacks and the screens at the top end. Thus, leakage passages are formed between adjacent preforms. The gas is then exclusively channeled towards only the interior passage of each annular stack at the bottom end. The top ends of the stacks are closed by solid screens. One disadvantage with this method is that the outer surfaces of the brake disks near the bottom of the stacks may become starved for gas, thereby producing an undesirable densification of the bottom brake disks and nonuniformity in densification between the bottom and top brake disks. Another disadvantage is that the closed top end of the stacks blocks the gas flow out of the top end, thus causing gas stagnation problems.
Another problem that often occurs during densification is soot and thick coatings on surfaces of the brake disks and tar on the furnace equipment. As is known to those in the art, soot usually refers to undesirable accumulations of carbon particles on the furnace equipment, while tar usually refers to undesirable accumulations of large hydrocarbon molecules on the furnace equipment. The large hydrocarbon molecules cause thick coatings on the surfaces of the brake disks. Typically, accumulations of soot and tar form when the reactant gas stagnates for a period of time in an area or comes into contact with cooler furnace surfaces. Stagnation typically occurs in areas where the gas flow is blocked or where the gas flow is moving more slowly than the surrounding gas flow.
Accumulations of soot and tar can cause a number of problems which affect both the quality of the carbon parts and the costs of manufacturing. Seal-coating is one typical problem that can result from soot and tar, although seal-coating can also be caused by other conditions that are described below. Seal-coating can occur when soot and large hydrocarbon molecules deposit excess carbon early in the densification process on surfaces of the porous structure. As the carbon accumulates on the surfaces of the porous structure, the surface pores eventually become blocked, or sealed, thus preventing the flow of reactant gas from further permeating the porous structure. As a result, densification of the interior region around the seal-coated surface prematurely stops, thereby leaving interior porous defects in the finished carbon part.
Maintenance costs also increase due to soot and tar accumulations on the furnace equipment. During the densification process, accumulations of soot and tar often form throughout the furnace equipment. As a result, an extensive manual cleaning process may be periodically required after each production run to remove all the accumulations and prepare the furnace for the next production run. This cleaning job can be very time consuming and can result in significant delays between production runs. The accumulations can also make disassembly of close fitting parts especially difficult since the accumulations tend to bind the parts tightly together. As a result, furnace equipment sometimes becomes damaged during disassembly due to the difficulty of separating the parts. Additionally, the furnace vacuum lines sometimes become constricted by soot and tar. As those in the art are familiar, the vacuum lines are used to generate the desired gas flow through the furnace. However, soot and tar accumulations sometimes build up in these lines and reduce the performance of the vacuum. Therefore, the vacuum lines must be regularly cleaned, which is a time consuming and expensive task.
In order to produce high quality, low cost parts, carbon deposition should be as uniform as possible around and through the porous structures. One way to achieve this desired uniformity is to optimize the residence time of the gas in the furnace. Residence time typically refers to the amount of time required for a gas to travel through the furnace or other designated area. Typically, a low residence time is associated with an unobstructed flow path and is generally preferred.
Gas flow obstructions often cause additional problems during densification. As previously mentioned, seal-coating is a common problem that causes porous defects within the interior region of the completed carbon parts. However, in addition to the causes previously described, seal-coating also can occur due to nonuniform carbon deposition. This typically occurs when a nonuniform gas flow accelerates carbon deposition at the surface of a part, thereby sealing the surface with carbon deposits and blocking gas diffusion into the interior of the carbon structure. Usually this type of seal-coating occurs later in the densification process when the density of the porous structures is higher.
Another problem associated with nonuniform carbon deposition is the formation of undesirable carbon microstructures. Depending on the process conditions in a CVI/CVD process, the deposited matrix can form different types of carbon microstructure, including rough laminar, smooth laminar, dark laminar, and isotropic. Rough laminar microstructure has the highest density and thermal conductivity, and the least amount of closed-off porosity (porosity that is unavailable for further matrix deposition via a CVI/CVD process.) Smooth laminar microstructure has lower density and thermal conductivity, and is harder than rough laminar. Isotropic carbon microstructure has the least desirable properties for use as a friction material. Dark laminar microstructure has properties between smooth laminar and isotropic microstructure. (For further discussion of the types of carbon microstructure, see H. O. Pierson and M. L. Lieberman, CARBON, Volume 13, 1975, pp. 159-166). A rough laminar carbon microstructure of the CVI deposited matrix is preferred because of the desirable friction and thermal characteristics of this microstructure. However, under certain process conditions, smooth laminar, dark laminar, and/or isotropic carbon microstructures may form instead. When making carbon-carbon composite friction materials, e.g. carbon composite brake disks, smooth and dark laminar and isotropic carbon microstructures within the CVI deposited matrix are generally undesirable because brake friction material disk performance is reduced.
Thus, previous processes, both isothermal and rapid densification processes, required multiple densification steps, with the porous structures requiring rearrangement and machining between steps in order to achieve acceptable densification results in the final product. The rearrangement and machining of the porous structure between cycles is costly and time-consuming. Thus, in spite of these advances, a non-pressure gradient CVI/CVD process and an apparatus for implementing that process are desired that quickly and uniformly densifies porous structures while minimizing cost and complexity. Such a non-pressure gradient process would preferably be capable of simultaneously densifying large numbers (as many as hundreds) of individual porous structures in a single step. In particular, a non-pressure gradient process is desired for quickly and economically densifying large numbers of annular fibrous preform structures for aircraft brake disks having desirable physical properties in a single cycle is preferred.